1. Field of the Invention
This invention relates, generally, to the art of concrete construction. More particularly, it relates to the testing of drilled shafts for defects.
2. Description of the Prior Art
Drilled shafts are large-diameter, cylindrically-shaped, concrete foundations cast directly into bored excavations having a depth of up to one hundred meters. They may be used as bridge piers or the like. Drilled shafts exhibit immense lateral stiffness and can be deeply embedded in the soil to overcome problems caused by scour (erosion aggravated by increased stream velocities around pier foundations).
However, imperfections in the concrete can greatly diminish the strength of a drilled shaft. Imperfections arise from the formation of foreign inclusions such as clumps of soil or pockets of drilling fluid that displace the concrete during the pouring process.
A typical drilled shaft has an upstanding cylindrical cage formed by steel reinforcing bars (rebars) embedded therein. The concrete radially inwardly of the cage is known as the core and the concrete radially outward of the cage is known as the protective cover. Imperfections in the core are of less concern than imperfections in the protective cover because if the protective cover is free of foreign inclusions, it will not erode under the harsh, aggressive environment of a bridge pier. Imperfections within the core are thus of little consequence. The thickness of the protective cover is directly related to its ability to stop corrosive elements from attacking the reinforcing steel. Further, both the steel and concrete must be present to provide structural integrity. The centrally positioned concrete core provides less lateral structural capacity than the more peripherally positioned concrete protective cover due to the relatively smaller moment of inertia associated with more centrally located areas. Core imperfections are therefore of less concern with regards to both structural capacity and corrosion protection.
However, if imperfections are in the protective cover, then such cover can be eroded away, thereby exposing the rebars. Corrosion then attacks the rebars and the ability of the drilled shaft to resist lateral bending is substantially diminished.
It is therefore important that the concrete used in a drilled shaft be substantially free of imperfections, especially in the protective cover. There are several well-known tests whereby the structural integrity of a drilled shaft can be tested after the concrete has cured, but such tests have the obvious shortcoming of being unavailable until the concrete has cured. If unacceptable imperfections are then detected, corrective action is difficult and expensive.
Another shortcoming of the known tests is that they best detect defects in the core of the drilled shaft. Defects in the protective cover are not reliably detected. Thus, the tests do a good job of detecting insignificant defects, but a poor job of finding critical defects. Specifically, the known tests include cross-hole sonic logging and small strain sonic echo. The equipment used in these tests attempts to locate anomalies in the cross-section of drilled shafts by measuring arrival times of lateral compression waves from between cast-in-place longitudinal pipes (C.S.L.), or from axial compression waves emitted from a small hammer at the shaft top (S.I.T.). Both methods provide useful information, but provide little useful information concerning anomalies positioned radially outwardly of the rebar cage, i.e. , in the protective cover where information concerning anomalies is most needed.
The C.S.L. method clearly delineates anomalies that appear between logging tubes but relies upon subsequent arrival times to estimate the condition surrounding the most direct path of the lateral compression waves. S.I.T. methods produce a qualitative estimate of the integrity of the shaft cross-section, but only the depth of the imperfection can be determined from such qualitative information. Thus, the radial position of the imperfection is unknown, thereby making remedial efforts difficult. Again, both the C.S.L. and the S.I.T. methods cannot be performed until after the concrete has cured.
A means for testing the structural integrity of the concrete before it has cured is needed. If such a means could be found, then the detected imperfections could be corrected before the concrete cures. The needed method would not only identify the depth of each imperfection, but its radial location as well. Such information would greatly facilitate remedial efforts.
An ideal method for detecting the presence of anomalies would detect anomalies in the core and in the protective cover of the drilled shaft prior to curing of the concrete.
The ideal method would further provide information as to the location of each anomaly.
However, in view of the prior art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the art how concrete could be tested for imperfections before it has cured, nor was it obvious how the location of such imperfections could be determined or how imperfections in the core and in the protective cover of a drilled shaft could be detected.
The long-standing but heretofore unfulfilled need for a concrete testing method having utility with uncured concrete is now met by a new, useful, and nonobvious method for detecting and locating foreign inclusions in a drilled shaft. At least one logging tube is positioned within a cylindrical drilled shaft in parallel relation to a longitudinal axis of the drilled shaft. Concrete is poured into the drilled shaft, covering the longitudinal extent of the at least one logging tube. The uppermost end of the at least one logging tube is not covered with concrete so that a temperature sensing means may be introduced into the lumen of said at least one logging tube.
The temperature sensing means is positioned within and is adjustable along the length of the at least one logging tube, i.e., the temperature sensing means is adjustable longitudinally within the at least one logging tube. By moving a temperature sensor from one longitudinal position to another, temperatures associated with a plurality of longitudinally spaced apart temperature locations within the at least one logging tube are sensed.
The structure having at least one logging tube includes a structure having a plurality of logging tubes where each logging tube is substantially parallel to a longitudinal axis of the drilled shaft.
Each temperature sensing means is also adapted to sense temperature in a radial direction relative to a longitudinal axis of a logging tube within which the temperature sensing means is positioned.
Accordingly, each temperature sensing location of a plurality of temperature sensing locations is determined by the location of the logging tube, the longitudinal position of the temperature sensing means along the length of the logging tube, and the radial position of the temperature sensing means relative to the longitudinal axis of the logging tube.
The temperature sensed at each of the temperature locations during the hydration phase of the concrete curing is monitored in real time. A temperature profile of the drilled shaft is thus generated, with temperature anomaly (low or high) readings indicating the presence of foreign inclusions. More particularly, a temperature reading that differs from an expected temperature is deemed to indicate the existence of a foreign inclusion. Significantly, the presence of a foreign inclusion is detected prior to full curing of the concrete.
A cylindrical rebar cage is positioned concentrically within the drilled shaft and a plurality of logging tubes are located in predetermined positions within the core or within the protective cover, or both. The spacing and number of logging tubes is selected such that the frequency of tubes provides sufficient information to detect defects within both the core and the protective cover.
An important object of this invention is to provide a concrete testing method having utility with uncured concrete.
Another important object is to provide a method that determines the integrity of concrete in the protective cover of a drilled shaft.
Still another object is to provide a method that closely identifies the location of a foreign inclusion both by its depth from the surface and by its radial position relative to a longitudinal axis of symmetry of a drilled shaft.
These and other important objects, advantages, and features of the invention will become clear as this description proceeds.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts that will be exemplified in the description set forth hereinafter and the scope of the invention will be indicated in the claims.